If a transmitter, in particular a wireless transmitter transmits a baseband signal from a digital modulator through an analog section and RF section to the antenna, impairments such as amplitude and delay mismatch may occur which in turn can lead to distortion. A delay mismatch between the in-phase and the quadrature component of the transmit signal may result from systematic sampling-time errors in a digital-to-analog converter DAC or phase misalignment in the low pass post-filters due to process drift or design asymmetry. Similarly, in systems with polar loop modulators a delay mismatch between amplitude and phase may be present due to a misalignment in the analog components.
An image rejection, which constitutes a measure of signal quality in GSM/EDGE systems, can be used to accurately subsuming an amplitude as well as a phase distortion. For GSM/EDGE baseband transmitters the lower limit of image rejection, which is commonly around 40 dB at a frequency of 67 kHz, can be derived among others from the limits of the GMSK phase errors, EDGE error vector magnitude and signal amplitude ripple as described in 3GPP TS 45.05, Radio transmission and reception, V4.3.0, Release 4, 3GPP, April-2001.
EP 1 376 567 relates to a compensation of an amplitude and phase mismatch in the baseband part of a receiver for quadrature modulated signals. Estimates for those distortions are derived by measuring the power and correlations in the baseband signal resulting from the injection of a calibration signal into the RF front. U.S. Pat. No. 6,670,900 relates to a compensation for an amplitude and phase mismatch in the baseband part of a transmitter for an OFDM (orthogonal frequency division multiplex) signal. Estimates for the amplitude and phase mismatch are derived for each single sub-carrier by means of calibration signals. However, none of these documents relate to compensation of a delay mismatch.
The one way to solve the problem of a delay mismatch is to design the analog signal path such that the limits of the design are fulfilled, i.e. all components have very tight variations relatively to each other such that the mismatch doesn't exceed certain thresholds. Such an approach is the standard approach for the transmit path of a base band section in mobile communication systems. However, as it is usually quite difficult within a mass production to fulfill these requirements, other approaches are required.